Springs for use as flexible support elements in support structures such as seating and bedding and furniture have traditionally and conventionally been constructed of spring steel and wire. See, for example, U.S. Pat. Nos. 188,636; 488,378; 1,887,058; 4,535,978; 4,339,834; 5,558,315. Attempts have been made to construct spring support elements out of plastic material. See, for example U.S. Pat. Nos. 4,530,490; 4,736,932; 5,165,125 and 5,265,291. Although fiber reinforced plastic springs are fairly well-developed, the use thereof in flexible support structures such as seating, furniture and bedding presents the formidable engineering challenge of providing suitable means for attachment of the springs to a frame structure and an overlying support surface. Plastic springs have heretofore been simply mechanically attached to a supporting structure such as described in U.S. Pat. No. 4,411,159 on a fiber reinforced plastic leaf spring for a vehicle. Any type of mechanical attachment is complicated by the extreme hardness and stiffness of fiber reinforced plastics. Ultimately it is nearly always necessary to drill attachment holes in the spring for a mechanical fastener (such as described in U.S. Pat. No. 4,736,932) requiring additional manufacturing and assembly steps. Also, drilling through the fiber-reinforced structure breaks the preferred long strand/roving fibers which are critical to providing optimal spring characteristics. The related application discloses clips for attachment of mattress foundation springs to a frame and an overlying grid. Although fully operative and novel, this approach requires additional parts and increased assembly tasks, and does not entirely overcome the negatives of possible slippage between the spring and the clips, and noise generation by such relative motion.
Conventional bedding systems commonly include a mattress supported by a foundation or "box spring". Foundations are provided to give support and firmness to the mattress as well as resilience in order to deflect under excessive or shock load. Foundations are typically composed of a rectangular wooden frame, a steel wire grid supported above the wooden frame by an array of steel wire springs such as compression type springs which are secured to the wooden frame. In order to properly support and maintain the firmness level in the mattress, a large number of compression springs are needed in the foundation, resulting in high production cost. This is the main disadvantage of using compression springs in mattress foundations. Also, foundations which use compression springs typically have a low carbon wire grid or matrix attached to the tops of the springs. Both the wires and the welds of the matrix can be bent or broken under abusive conditions. In such steel/metal systems, fasteners are required to secure the springs to the grid and to the frame. This leads to metal-to-metal contact which can easily produce squeaking sounds under dynamic loading.
In an effort to avoid the high cost of using compression springs in foundations, another type of spring used is the torsional steel spring formed from heavy gauge steel spring wire bent into multiple continuous sections which deflect by torsion when compressed. See for example U.S. Pat. Nos. 4,932,535; 5,346,190 and 5,558,315. Because torsional springs are dimensionally larger and stiffer than compression springs, fewer torsional springs are needed in the foundation. However, the manufacture of torsional-type springs from steel wire requires very expensive tooling and bending equipment. Elaborate progressive bending dies are required to produce the complex torsional spring module shapes which may include four or more adjoining sections. The manufacturing process is not economically adaptable to produce different spring configurations without new tooling, tooling reworking and/or machinery set-up changes and process disruption, etc. Therefore, the configuration and resultant spring rate of such springs cannot be easily or inexpensively altered to produce foundations with different support characteristics. Furthermore, the many bends in these types of springs make dimensional quality control and spring rate tolerance control very difficult to achieve. Also, variations in steel material properties and the need for corrosion protection and heat-treating add to the cost and difficulty of producing steel wire spring modules. And furthermore, the awkward geometry of the relatively large torsional springs makes assembly of the springs in the foundation frame relatively difficult.
Another disadvantage of the use of steel wire springs in foundations, and a particular disadvantage of torsional springs, is the phenomenon of "spring set" in which a spring does not return completely to an uncompressed height following excessive loading. So long as a spring is deflected within its spring rate tolerance range, it can be repeatedly loaded for a certain number of cycles without noticeable change in operating characteristics. However, if deflected past the maximum deflection range, it will undergo permanent deformation or "set", resulting in a permanent change in operating characteristics such as lack of reflexive support, permanent change in shape, or catastrophic failure in the form of breakage. Spring set in steel wire springs may also occur simply following prolonged normal use, i.e., continuous heavy loading. This phenomenon is also generally referred to as fatigue and can result in catastrophic failure.
Mattresses of increased thickness dimension such as "pillow-top" mattresses, when placed on top of traditional foundations of six to eight inch height, can be too high in proportion to the head and foot boards of beds, resulting in an awkward appearance and an excessively high sleeping surface. This trend toward larger mattress and foundations increases distribution and storage costs. Mattress foundations in the United States typically measure on the order of five to eight inches thick, with an average thickness (or height) of six and one half to seven and one half inches. In conventional foundations, most all of this dimension is attributable to the height of the wire spring modules. In general, deflection of torsional wire spring modules is limited to approximately 20% of the total height dimension. Compression which exceeds the 20% range can cause spring set or breakage. Reducing the overall height of torsional spring modules can make the springs too rigid and diminishes the desired deflection characteristics and ability to absorb heavy loads with recovery. Moreover, the number of cycles to failure during life testing is generally harder to predict with shortened height spring wire modules and is usually many less cycles to failure than spring wire modules of greater height. Nonetheless, it would be desirable to have a foundation with reduced height while retaining the desired support and deflection characteristics.